Two Ukrainian air bases in Crimea have been overrun by pro-Russian forces.

Pro-Russian forces appeared in control of the base in Belbek, near Sevastopol, after an armored vehicle was seen ramming into the gate of the base and an ambulance drove into the compound at high speed.

A live video feed from outside the gate showed men in civilian clothes entering the base. There were also reports of explosions and gunfire.

The events occurred after a Russian ultimatum to the personnel at Belbek to surrender the base.

The Ukrainian commander of the base, Yuliy Mamchur, said there was at least one injury.

Mamchur said he summoned his men together, sang the Ukrainian national anthem, and then stood at ease. He said they were going to turn over their weapons.

Meanwhile, Russia's Defense Ministry said that as of March 21, the Russian state flag had been raised at 147 Ukrainian armed forces' facilities in Crimea. Russian Navy flags were raised on 54 Ukrainian naval ships, including Ukraine's only submarine.

In the eastern Ukrainian city of Donetsk, some 5,000 people attended a pro-Russian rally, chanting slogans in favor of a referendum on seceding from Ukraine.

Donetsk resident Galina Zakazchikova said she was against the new Ukrainian authorities.

"Look what is happening in the country, it is a coup d'etat, it is against the law, and we are against these authorities," she said. "We are for the legitimate President [Viktor] Yanukovych, and we are waiting for him."

Russian President Vladimir Putin signed a law formalizing Crimea's annexation on March 21, despite U.S. and European sanctions.

Speaking in Kyiv on March 22 after meeting with Ukraine's acting President Oleksandr Turchynov, Steinmeier said he hoped the first Organization for Security and Cooperation in Europe (OSCE) monitors would arrive in Ukraine to support de-escalation efforts in the next couple of days.

Moscow has slammed the second wave of sanctions imposed by the European Union over the Crimean crisis as "divorced from reality."

On March 21, the EU added 12 Russians and Ukrainians to a list of people targeted by asset freezes and travel bans, bringing to 33 the number of figures targeted by the European bloc.

Russian Foreign Ministry spokesman Aleksandr Lukashevich said on March 22 that Russia reserved the right to "give a comparable answer to the actions taken."

In a separate statement, the ministry expressed hope that the decision to send international monitors to Ukraine will help "overcome the internal Ukrainian crisis, stop rampant nationalist banditry, [and] eradicate ultra-radical tendencies."

Russia agreed late on March 21 to join the 56 other members of the OSCE in a consensus decision to send a monitoring mission to Ukraine. It added that the "mission's mandate reflects the new political and legal realities and does not extend to Crimea and Sevastopol, which have become part of Russia."

The OSCE had said the observer team -- numbering at least 100 -- will gather information over six months on the security situation "throughout" Ukraine.

U.S. chief envoy Daniel Baer said the observers will start deploying within 24 hours of the March 21 decision.